A conventional hybrid vehicle is comprised of an internal combustion engine and an electric motor having a generating function, and a power of the engine is distributed to the motor side and an output side connected to drive wheels through a power distribution device as a differential mechanism. One of rotary members of the power distribution device is connected to the engine, and another rotary member of the power distribution device is connected to the motor. A drive mode of the hybrid vehicle can be selected from engine mode where the vehicle is powered only by the engine, motor mode where the vehicle is powered only by the motor, and hybrid mode where the vehicle is powered by both engine and motor.
For example, US 2009/0082171 A1 describes a hybrid vehicle comprised of an engine and two motors. The hybrid vehicle taught by US 2009/0082171 A1 is provided with a planetary gear set including a ring gear being selectively connectible to the first motor/generator and the engine through a clutch or brake, a sun gear being continuously connected to the second motor/generator, and a carrier member continuously connected to drive wheels through an output member. According to the teachings of US 2009/0082171 A1, the transmission includes clutches and brakes that are selectively engageable alone or in different combinations to connect different ones of the nodes to one another or to a stationary member, thereby establishing the various operating modes. Specifically, the operation mode is shifted among the two motor electric-only mode where both motor/generators are operated, the one motor electric-only mode where only the second motor/generator is operated, and the output split mode where the engine and the second motor/generator are operated.
Another example of the hybrid vehicle is described in Japanese Patent Laid-Open No. 08-295140. According to the teachings of Japanese Patent Laid-Open No. 08-295140, a planetary gear unit serves as the power distribution device in which a carrier is connected to an engine, a sun gear is connected to a generator/motor, and a torque of an electric motor is added to a torque of a ring gear. To this end, the engine and the carrier are selectively stopped by the brake. The power distribution device is used as a speed reducer by stopping the carrier by the brake when the vehicle is powered by the generator/motor while stopping the engine.
Thus, according to the teachings of the foregoing prior art documents, the drive mode is shifted by altering engagement states of the clutch and the brake. However, since output performances of the power units activated before and after the mode shifting are different, required drive force may not be achieved. That is, it is difficult to shift the drive mode promptly while avoiding shortage of the drive force. For example, in case of shifting from the motor only mode to the engine mode to establish large drive force, the motor has to be used to start the engine. In this case, the required drive force may not be achieved as a result of thus consuming the power of the motor to start the engine.